This invention relates to a suspension control system for use with an automotive vehicle supported on front and rear pairs of road wheels to control the damping force characteristic of each of the shock absorbers interposed between a sprung mass (vehicle body) and an unsprung mass (corresponding one of the road wheels).
For example, Japanese Patent Kokai Nos. 61-163011 discloses an automotive vehicle suspension control system for controlling the damping force characteristics of the shock absorbers provided on an automotive vehicle. The conventional suspension control system is arranged to suppress vibrations on the vehicle body by controlling the shock absorber damping force characteristics toward the hard side when both of the sprung mass vertical velocity and the relative velocity between the sprung and unsprung masses have the same sign and to suppress the unsprung mass input to be transmitted to the sprung mass by controlling the shock absorber damping force characteristics toward the soft side when the sprung mass vertical velocity and the relative velocity between the sprung and unsprung masses have different signs. The sprung mass vertical velocity is derived from the sprung mass vertical acceleration sensed with the use of a vertical acceleration sensor mounted on the sprung mass (vehicle body), and the relative velocity is derived from the relative displacement sensed by a stroke sensor. Thus, the conventional suspension control system requires such a stroke sensor in addition to the vertical acceleration sensor, resulting in a complex and expensive suspension control system. Furthermore, the signal indicative of the derived relative velocity contains components of unsprung mass resonance frequency (10 to 15 Hz) and the resulting control signal used in operating the corresponding shock absorber is taken in the form of a high frequency signal with such a high frequency control signal, the shock absorber is required to be associated with a very expensive actuator operable in fast response to the control signal.